Zinc oxide



prl 7, 1936. E, H. BUNCE ET A1.

ZINC OXIDE Filed March 24, 1935 ATTORN EY Patented Apr. 7, 1936 UN ITED STAT ES PATENT OFFICE ZINC OXIDE Jersey Application March 24, 1933, Serial No. 662,518

2 Claims.

This invention relates to the manufacture of zinc oxideand has for its object the provision of certainimprovements in zinc oxide manufacture. Aparticular aim of the invention is the provision of certain improvements in the manufacture of zincv oxide from zinc vapor resulting from the reduction of a porous charge of agglomerates of mixed' zinciferous and carbonaceous materials in an upright or verti-cal reducing chamber.

Zinc oxide is commonly made by the oxidation or combustion of metallic zinc vapor. Atmospheric air is the ordinary oxidizing agent, and the resulting fume of zinc oxide particles in suspension in air is drawn or forced through a flue system to a bag-room or other appropriate zinc oxide collecting apparatus, air being thus used for the pneumatic transportation of the zinc oxide particles through the flue system.

It has heretofore been recognized that a certain amount of gas, such as air, exhaust gases, etc. may be introduced with advantage into the residue discharge or lower end of an upright or vertical reducing chamber during the reduction of a porous charge of agglomerates. To this end, the agglomerates are of such a character that they do not break down or -disintegrate into fines during the reducing operation, so that gas may be introduced through the residue about to be discharged, either by the suction due to the stack draft of the retort (with the bottom of the retort open) orby injection under pressure (with the bottom of the retort sealed). The main advantage oi introducing appropriate amounts of gas into the lower end of the upright reducing chamber is the prevention of loss of zinc in the discharged residue. Zinc vapor tends to diffuse downwards in the retort and condense on the cooling residue about to be discharged. The introduction of gas in amounts suflicient to create an upward gas current of sunicient velocity to counteract this tendency of the zinc vapor to diiusev downwards thus results in keep-ing the residue free from zinc that would otherwise condense therein and be lost.

The amount'of gas introduced at the bottom of the reducing chamber must be carefully regulated. If too little gas be introduced, the velocity ofthe ascending gas current will not be suilicient to prevent the diffusion of zinc vapor into the residue. The introduction of an yover-supply of gases containing oxidizing constituents such as air or CO2 may develop so much heat as to fuse ormelt the charge, causing it to run out of the bottom vofthe reducing chamber. Such fusion of 1 the charge is accompanied by loss of zinc and injury to the reducing chamber. Even though the charge merely becomes pasty, there results a closing up or diminution of the voids between the agglomerates, thereby lowering the permeability of the charge to gas ilow and thus retarding effective reduction. Furthermore, too much gas (even if the gas be neutral or reducing) may result in such an excessive dilution of the zinc vapor evolved from the retort as to have an undesirable effect on the zinc oxide product, notably imparting to it too little hiding power or body for use in paint. A very large excess of oxidizing gases may cause part of the zinc vapor to be burned to zinc oxide in the reducing chamber instead of in the contemplated combustion or oxidation chamber or zone, in which case the zinc oxide may be rendered gritty or discolored by overheating in the retort. The introduction of excess gas at the bottom of the reducing chamber may set up such a strong current of gas through the chamber as toy entrain particles of the charge which mingle with the zinc oxide product and render it gritty and specky.

In making zinc oxide from zinc vapor derived from the reduction of a porous charge of agglomerates in an upright reducing chamber, we have found that the suction necessary for withdrawing the oxide from the oxidizing or combustion zone and for transporting it to the collecting apparatus exercises a marked effect on the pressure prevailing in the reduction chamber, and therefore on the gas current owing therethrough. In case the bottom of the retort is open, the effect of such suction is ordinarily to augment or supplement the normal stack-draft effect of the reducing chamber, with consequent increase in the amount of gas entering at the bottom. In case the bottom of the retort is sealed, the effect of such suction is to create a vacuum in the retort that tends to facilitate the inflow of injected gas, and even of combustion gases from the heating chamber surrounding the retort, if the wall of the retort is permeable to gases on account of the presence of cracks. The inflow of such combustion gases exerts a corrosive eiiect on the retort wall and may enlarge minute cracks to dangerous leaks.

We have now discovered that the pressure in the retort and the ascending gas current therein may be controlled, with consequent regulation of the amount of gas introduced at the bottom of the chamber, by appropriately regulating and controlling the eifect thereon of the current of air supplied to burn the zinc vapor to zinc oxide and to transport the zinc oxide to the collecting apparatus.

Based on these considerations and discoveries, our present invention, in its broad aspect, involves blowing or otherwise appropriately admitting air in controlled amount into the zinc oxide flue system for promoting the transportation there-Y through of the fume. More particularly, the invention involves, in making zinc oxide from zinc vapor resulting from the reduction of a porous charge of agglomerates in an upright reducing chamber, control of the amount of gas introduced into the lower end of the reducing chamber by regulating (1) the air supplied (by blasting or otherwise) for the oxidation of the zinc vapor or (2) the air admitted to the flue system for promoting the pneumatic transportation of the oxide therethrough, or both. In the practice of the invention in its complete aspect, there is a threefold control of the gas and air supply:

(l) Gas or primary air is introduced at the bottom of the reducing chamber in amounts adequate to prevent condensation of zinc in the residue without injuring the quality of the zinc oxide produced.

(2) Secondary air is supplied to the current of zinc vapor from the reducing chamber to burn it to zinc oxide.

(3) Tertiary air is supplied to the zinc oxide formed in order to facilitate its pneumatic transportation to the bag-room.

' Part of the tertiary air may be supplied together with the secondary air at the same inlet or inlets; or all of the tertiary air may be admitted either by blowing or by suction at an inlet or inlets separate from the inlets supplying the secondary air, and located so far from the inlets of the secondary air as not to affect the flame with which the zinc Vapor burns to zinc oxide.

Fig. 1 of the accompanying drawing diagrammatically illustrates an apparatus adapted for the practice of the invention, and

Fig. 2 illustrates a modified form of apparatus With which the invention may be practiced.

Referring to the drawing, the reducing chamber is shown as a vertically disposed and externally heated retort IIl surrounded by a heating chamber or laboratory Il of a furnace structure IZ. The upper end of the retort I0 preferably extends a substantial distance above the upper end of the furnace structure and constitutes a prolongation I3 in which purication of the zinc vapor may be carried out as described in United States Patent No. 1,749,127, March 4, 1930, whereby a relatively pure (lead-free) Zinc oxide pigment may be made when desired. The lower end of the retort I0 joins a downwardly extending charge coni-"ming member I 0a which dips into a liquid-seal 35 that provides a. gas-tight discharge for the solid residue. The reduced charge may be regulated in its passage through the retort I0 and the extension Illa by means of a discharge rotor 36. Tuyres 37, attached to appropriate supply pipes not shown, are provided just above the discharge rotor for introducing exhaust gases, steam or primary air, in regulated amounts, into the retort.

An off-take pipe or conduit I4 extends laterally from the top of the retort or its prolongation for the withdrawal of the gaseous products of the reducing operation, principally metallic zinc' vapor and carbon monoxide. A charging well IEis provided within the upper end of the prolongation I 3 and extends a short distance below the opening of the off-take pipe I4 thereby avoiding obstruction of the latter by the charge. In the drawing,

the charging well I5 is shown closed by a removable cap I5. When the reducing operation is conducted as a continuous process, the cap I6 is replaced by an appropriate charging device for delivering charge to the retort as required while maintaining the top of the retort closed or sealed for all practical purposes. n Y

The off-take pipe I4 has a vertically disposed zinc vapor outlet I1 in its top wall. A nozzle I8, registering with the outlet Il, is mounted above the outlet and is surrounded by an air or windbox I9 having an annular slot 20 adjacent to and surrounding the mouth of the nozzle I8.

A pipe 2| supplies compressed air from a blower or air compressor 22 to the interior of the windbox I9. The pipe 2l is provided with a slide damper 23 for regulating and controlling the air delivered to the wind-box. An OIT-take flue 24 is operatively mounted above the wind-box I9 for conveying the fume of suspended zinc oxide particles to the bag-room or other zinc oxide co1- lector. A suction fan 25 is included in the flue 24. The iiue system comprises thenecessary series of iiues and suction fans for withdrawing the zincoxide from the zone of combustion and pneumatically transporting it to the zinc oxide collector.

Air for promoting or for facilitating the transport of the Zinc oxide through the flue system is supplied under pressure to the flue 24 by a pip-e 28 connected to the blower or compressor 22. The pipe 26 has a slide damper 21 for regulatingand controlling the amount of air thus supplied to the flue 24. 'Ihe pipe 26 communicates with the iiue 24 at a point so far removed from the zone of combustion or oxidation as not to affect the flame with which the zinc vapor burns to zinc oxide.

In the operation of the apparatus of Fig. 1, the vertical retort I0 and its prolongation I3 are kept fullv of the charge of non-disintegrating agglomerates of zinc ore and reduction fuel, fresh agglomerates being introduced through the charging well and spent residues being withdrawn from the bottom of the retort as required. Gas (such as exhaust gas) or primary air is admitted at the bottom of the retort to prevent condensation of zinc in the residue. A circumferential blast of secondary air is directed against the streamrof zinc vapor and carbon monoxide issuing from the nozzle I8. The resulting particles of zinc oxide are suspended in the resulting gases (for the most part, carbon dioxide, nitrogen and such excess of air as may be supplied by the blast) and are withdrawn from the zone of combustion or oxidation (at the lower end of the flue 24) by the suction of the fan 25 and transported through the ue system to the bag-room or other collecting apparatus. Tertiary air for promoting the transport of the zinc oxide particles through the flue system is supplied to the flue 24 by the pipe 26.

The amount of gas or primary air entering the bottom of the retort is determined by the following factors: f

(1)a In the case Vof a retort with open bottom, the natural stack-draft of the retort and vits appurtenances (determined by its permeability to gases, temperature, and the tendency'of the zinc vapor and carbon monoxide evolved therein to set up a back pressure). Y

(1)1) In the case of a retortwith closed bottom, the pressure at which gas or primary air is supplied by the tuyres 3l at the bottom of the retort.

(2) The amount of suction developed by the fan 25.

(3) The volume and rate of air introduction through the pipe 2|.

(4) The volume and rate of air introduction through the pipe 26.

The relative amounts of air supplied by the pipes 2| and 2S will be largely determined by the 'contemplated character of the operation. In

general, the greater the amount of air introduced through the pipe 2|, and hence directed as a blast against the zinc vapor, the greater will be the dilution and the more sudden the cooling of the resulting zinc oxide particles, and hence the ner or smaller will be the average particle size of the resulting zinc oxide product. The contemplated characteristics of the zinc oxide product will therefore determine in a large measure the amount of air to be supplied for blasting through the pipe 2|. Since, in practice, it will ordinarily be desirable to maintain this factor substantially constant during the production of any particular grade of zinc oxide, the control of the amount of gas or primary air admitted at the bottom of the retort will be principally effected by regulation and control of the pressure at the tuyres 3`| and of the tertiary air supplied by the pipe 26.

As has hereinbefore been pointed out, the invention contemplates introducing the gases which pass into the retort IU either by means of a tuyre, or, by merely opening the bottom to allow the gas to` be drawn in. Accordingly, in such modication of operation which is characterized by the introduction of gas through an open bottom, the use of the tuyres may be wholly or partially discontinued and all or a part of the gas introduced or admitted through the open bottom. In one method of operating with an open bottom, the sealing liquid in the seal 35 may be removed and the primary gas will then have free entrance to the lower extension of the retort. However, the sealing apparatus may be Wholly dispensed with and the lower portion of the retort left open to admit air or other gas as conditions of operation may dictate.

It will be noted that in the apparatus oi'f Fig. 1, all of the secondary and tertiary air is supplied by the blower or air compressor 22. By ltering or otherwise appropriately cleaning the air supplied to the blower or compressor, contamination of the zinc oxide product by such substances as dust, soot and the like ordinarily suspended in the atmosphere around a zinc oxide plant may be effectively prevented.

In the apparatus shown in Fig. 2 of the drawing, the suction of the fan 25 is utilized for drawing into the flue system both the secondary and tertiary air in controlled amount. The zinc vapor issuing from the outlet (of the off-take pipe I4) is oxidized or burned by air drawn into the lower end of the flue 24 through adjustable openings 28. A slide damper 29 is operatively associated with each opening 28 for adjusting the effective area of the opening. Tertiary air is admitted through openings 3|) provided with adjustable slide dampers 3|. In accordance with the invention, either or both of the series of openings 28 and 3!! is regulated and controlled, with respect to the effective area thereof, in such manner as to control the suction thereby applied to the retort.

The gases introduced into the bottom of the retort may be air, reducing gases (e. g., water gas), steam, nitrogen, carbon dioxide, etc.

We claim:

l. The method of making zinc oxide which comprises heating an elongated upright porous column of mixed and agglomerated zinciferous and carbonaceous materials through a Wall to produce zinc vapor, introducing gas into said column near its base to facilitate the passage of zinc vapor upwardly therethrough, withdrawing a gaseous stream containing zinc vapor from the upper end portion of the column, introducing a blast of oxidizing gas into the stream to produce zinc oxide, pneumatically transporting the resulting zinc oxide through a conduit to a collecting zone and collecting zinc oxide therein, and introducing a controlled amount of air into said conduit to facilitate the transport of zinc oxide therethrough and simultaneously to regulate the amount of gas introduced into the base of the column.

2. The method of making zinc oxide which comprises heating an elongated upright porous column of mixed and agglomerated zinciferous and carbonaceous materials through a 'wall to produce zinc vapor, introducing gas into said column near its base to facilitate the passage of zinc vapor upwardly therethrough, withdrawing a gaseous stream containing zinc vapor from the upper end portion of the column, introducing a blast of oxidizing gas into the stream to produce zinc oxide, pneumatically transporting the resulting zinc oxide through a conduit to a collecting zone and collecting zinc oxide therein, introducing a controlled amount of air into said conduit to facilitate the transport of zinc oxide therethrough and simultaneously to regulate the amount of gas introduced into the base of the column, and regulating the amount of oxidizing gas introduced into the stream of zinc vapor to regulate the amount of gas introduced into the base of the column.

EARL H. BUNCE. HARRY M. HASLAM. 

